I’ve found three extensions/libs that apparently could do the job for me (fullhistory, django-history and django-reversion) so I decided to test them, but because of the lack of structure/documentation of the other two I tried just the last one.

django-reversion

(http://code.google.com/p/django-reversion/)

It has a good documentation, integration with the admin and the most important for me a low level api, as I am not using the admin app for this project.

The installation went pretty well (svn co http://django-reversion.googlecode.com/svn/tags/1.1.2/src/reversion to your PYTHONPATH, add the reversion app to your INSTALLED_APPS in settings.py and the usual python manage.py syncdb).

To track the changes of a model you must register it with the reversion framework (importing reversion and calling reversion.register(YourModel)).

Reversion provides some ways for creating revisions of your model and the docs recommend that you choose one of them to use in your project. I chose the MiddleWare method because it seemed the better option for our project, but you should look the reversion documentation and decide which one is the best for you.

After registering the model and installing the MiddleWare no more changes are needed, whenever I save a model a new revision is created on the database. Another cool feature is the follow argument when registering a model. That way you can specify a ForeingKey/ManyToMany field to follow. This means that when you save the model a new revision is created with all “followed” fields in its current state. It’s very useful. See the docs for an example.

The only thing that’s not so clear for me is: what happens if I delete a field from a model that’s registered within reversion? Can you still restore its versions?

But enough of cheap talk, I’ll show you the code.

My first “useful” piece of code for Wave is a Tweet Gadget. It’s a simple Google Gadget that takes advantage of the Wave extensions for interacting with the Wave user. What the gadget does is very simple, it takes a tweet id and using the Twitter API it places the tweet inside the Wave. It’s a simple way to quote a tweet.

I must say that the original idea is not mine, it’s rbp‘s idea. We were chatting using Google Wave and he wanted to quote a tweet. That’s how we started looking ways of doing it.

Stateless gadget

I started writing a simple gadget that using the Google Gadgets API makes a request using the Twitter API and shows the tweet in the wave. The problem of this first version was that it does not keep the state, so if you reload/rejoin the wave, you won’t see the tweet, but instead you will see the text box for entering the tweet id.

In this first version you can see how to make a JSON Async Request using the Google Gadgets API. It’s very simple, all you need to do is to set the parameter gadgets.io.RequestParameters.CONTENT_TYPE to gadgets.io.ContentType.JSON and then call gadgets.io.makeRequest(url, callback, params) where callback is a function that receives the JSON object.A very simple example:


function myCallback(obj) {
jsondata = obj.data;
// access the attributes as jsondata['key']
// ...
}
function makesJSONRequest(url) {
params[gadgets.io.RequestParameters.CONTENT_TYPE] = gadgets.io.ContentType.JSON;
gadgets.io.makeRequest(url, myCallback, params);
}


Adding state

Until now the gadget is a simple Google Gadget as it does not use any feature of Google Wave. Also it’s not so useful because it can’t keep the tweet id when you leave the Wave. And worse, people will never see the tweet you quoted. One of the main differences between simple Google Gadgets and Wave Gadgets is their ability to keep, set and change state. By state I mean information. You can keep user preferences, a game score or a tweet id (or even a tweet).

I decided that the easiest way was to store the tweet as it will appear in the wave (with HTML entities and everything). The new version will keep the tweet information when the user enters the tweet id, so everybody in the wave can see the quoted tweet.

You can see the Gadget code to understand how state works in Google Wave, but what’s most important:

• use a callback (with wave.setStateCallback()) to be aware of state changes
• wave.getState() returns an state dict-like object which you can set and get information from
• use wave.getState().get(‘key’) for getting an information
• and wave.getState().submitDelta({‘key1:’ value1, ‘key2′: value2} for setting information

And that’s it. It’s very simple, hope you enjoy it

This weekend (October 3rd and 4th) will happen the first “Transparência HackDay” (“Transparency HackDay” in english), a free and open hacking event with focus on bringing together hackers, activists, managers and people with ideas to make government information/public data more accessible for everyone using and writing web tools.

I’m attending and one of my ideas is to hack CET (Companhia de Engenharia de Tráfego / Traffic Engineering Company) data and make them available. The idea is to improve a crawler I wrote (in python ) that downloads and stores data about the traffic in São Paulo every 30 minutes from the “traffic now” CET website (as they don’t store it [or at least don't provide anything]).

More information about the HackDay (in portuguese):
http://www.slideshare.net/esferamobi/transparencia-hackday-proposta